494 vs 1600: Why Delhi's AQI readings differ so drastically
What's the story
Delhi's air quality index (AQI) hit alarming levels on Monday, with the Central Pollution Control Board (CPCB) recording an AQI of 494.
This was the second-highest daily reading by the CPCB since it was established in 2015.
However, international platforms like IQAir reported even higher figures, crossing 1,600 in the afternoon.
Here's why.
Methodology
Understanding the difference in AQI readings
The Indian AQI scale is capped at 500, with values between 451-500 categorized as "severe plus." This threshold was selected as health impacts plateau beyond this point.
Dipankar Saha, ex-head of CPCB's Air Laboratory Division, said giving AQI values above 500 could help highlight the severity of an air crisis.
IQAir, on the other hand, follows the United States Environmental Protection Agency's model which has no upper limit and categorizes any value above 301 as "hazardous."
Calculation
Different formulas lead to varying AQI data
The Indian and US scales use different formulas to calculate the AQI, resulting in different data for the same pollutants.
On Monday, while India's AQI hovered around 490, IQAir's index reported values above 1,000.
However, despite the differences, both readings reflected extremely hazardous air quality conditions in Delhi.
The Indian AQI is more accurate as it is analyzer-based unlike IQAir's sensor-based index.
Pollution surge
Delhi's pollutant concentrations reach alarming levels
Pollutant concentrations such as PM10 and PM2.5 are measured in micrograms per cubic meter without an upper limit.
On Monday, Delhi's PM2.5 concentration hit a 24-hour average of 422.8 ug/m3, up from 305.5 ug/m3 a day earlier.
This sharp spike further highlighted the severity of the air quality crisis in the city, irrespective of the discrepancies in AQI readings between Indian and international platforms.